The Y-value defined by the curve is the factor output.
For the curve controls see: Curve widget.

Tip

Flipping the curve around reverses the time input, but
doing so is easily overlooked in the node setup.

Start, End

Start frame and End frame of the range of time specifying the values
the output should last. This range becomes the X-axis of the graph.
The time input could be reversed by specifying a start frame greater than the end frame.

A speed of time factor (from 0.00 to 1.00) relative to the frame rate
defined in the Render Dimensions Panel.
The factor changes according to the defined curve.

Hint

Output values

The Map Value
node can be used to map the output to a more appropriate value.
With sometimes curves, it is possible that the Time node may output a number larger than one or less than zero.
To be safe, use the Min/Max clamping function of the Map Value node to limit output.